At the engine constituting an internal combustion engine, the rotational speed of the engine in the so-called idle drive (hereinafter, also referred to as “idle rotational speed”) in which a self-sustained operation is conducted in a state where the driving force is not transmitted to the load after the engine has been started is desirably set as low as possible in a range where self-sustained operation is allowed for the purpose of reducing fuel consumption.
During the operation of the engine, vibration will occurs thereby. The idle rotational speed is set higher than the rotational speed at which resonance of the driving force transmission system including the engine occurs (hereinafter, also referred to as “resonant rotational speed”) for the purpose of reducing vibration during idle operation.
Japanese Patent Laying-Open No. 2006-152877 (PTL 1) discloses a hybrid vehicle that has the mounted engine started by cranking through a motor, in which the motor is configured to set the engine rotational speed lower than the resonant rotational speed when there is a possibility of matching the resonant rotational speed of the driving force transmission system at the time of cranking due to suppressing the increase of the engine rotational speed.
According to the configuration disclosed in Japanese Patent Laying-Open No. 2006-152877 (PTL 1), resonance at the driving force transmission system can be suppressed even in the case where there is a possibility of the engine rotational speed matching the resonant rotational speed due to reduction of the motor output caused by increase of friction torque or lower battery output at the time of cranking during engine starting.